FIG. 1 shows a string tuning device for a stringed instrument disclosed by the U.S. Pat. No. 4,827,825. The string tuning device is installed on an instrument such as a guitar, and adjusts the tightness of a string 4 to perform tuning. The string tuning device includes a sleeve 1. An internal thread part 2 and a core shaft 3 are respectively disposed at two ends of the sleeve 1, and a worm gear (not shown in FIG. 1) may be driven by rotating a worm knob (not shown in FIG. 1). Thus, the core shaft 3 is driven and rotated to change a distance between the internal thread part 2 and the core shaft 3, such that one end of a string 4 may pass the sleeve 1, and the sleeve 1 is rotated to cause the core shaft 3 and the internal thread part 2 clamp the string 4. The other end of the string 4 is fixed on the stringed instrument. Accordingly, when the string 4 is tensed and one end of the string 4 is clamped by the core shaft 3 and the internal thread part 2, the worm knob may be rotated to drive the sleeve 1 to rotate, thereby performing tuning through changing the tightness of the string 4.
The above conventional string tuning device allows the string 4 to be fixed on the sleeve 1 through a simple winding approach, and provides a self-locking function. However, when the string 4 is clamped by the core shaft 3 and the internal thread part 2, a lateral shearing and stretching force is generated upon the string 4 because the core shaft 3 presses tightly against the string 4 through rotation, hence significantly increasing the probability of breaking the string 4. Further, in this conventional string tuning device, to unlock the string 4, as the core shaft 3 has usually reached its minimum secure fastening torque and is thus dead locked, a word screwdriver is needed to first loosen the internal thread part 2 in order to separate the core shaft 3 and the internal thread part 2 and then unlock the string 4. Thus, operations of such conventional string tuning device are quite complicated.